Precipitated silicas and/or silicates find use in a broad range of manufactured products ranging from cosmetic and food products to industrial coatings (such as for paper as one example) and elastomeric materials, such as tires. Silicas are particularly useful in dentifrice products (such as toothpastes) where they function as fillers, abrasives, and thickeners, as well as anti-caking agents and glidants for food and pharmaceutical uses. Because of this functional versatility, and also because silicas have good cleaning ability, are relatively safe, and have high compatibility with typical dentifrice ingredients like humectants, thickening agents, flavoring agents, and therapeutic agent such as anti-caries agents, there is a strong desire among toothpaste and dentifrice formulators to include them in their products. Silicates are utilized as active ingredients, such as oil absorbers and odor absorbers, as some examples, as well as additives for various physical and chemical purposes, such as viscosity modification, again, as one example within a number of different formulations, such as personal care compositions, antiperspirants, and other like products, and as paper coating agents and/or anti-caking agents. Such silicates (for example, sodium aluminosilicate) exhibit effective thickening, absorption, etc., characteristics as well as effective compatibility with much the same added ingredients listed above for silicas.
As known, synthetic precipitated silicas generally are produced by the de-stabilization and precipitation of amorphous silica from soluble alkaline silicate by the addition of a mineral acid and/or acid gases under conditions in which primary particles initially formed tend to associate with each other to form a plurality of aggregates (i.e., discrete clusters of primary particles), but without agglomeration into a three-dimensional gel structure. The resulting precipitate is separated from the aqueous fraction of the reaction mixture by filtering, washing, and drying procedures, and then the dried product is mechanically comminuted in order to provide a suitable particle size. Silicates are produced generally in the same manner by reacting sodium silicate with appropriate reactants to produce the desired silicate; i.e. calcium hydroxide to produce calcium silicate, magnesium hydroxide to produce magnesium silicate, aluminum sulfate replacing the mineral acid to produce an aluminosilicate. Mixed metal silicates can likewise be produced, i.e. sodium magnesium aluminosilicate by reacting sodium silicate, magnesium hydroxide and aluminum sulfate, or calcium magnesium silicate by reacting sodium silicate, calcium hydroxide, and magnesium hydroxide, as non-limiting examples.
The silica and/or silicate drying procedures are conventionally accomplished using spray drying, nozzle drying (e.g., tower or fountain), flash drying, rotary wheel drying, oven/fluid bed drying, and the like, which often require considerable expenditures for equipment and operating costs. The conventional drying procedures tend to reduce the ability to control final product physical properties and increase the chance of contamination. A similar issue is associated with other synthetically derived polishing agents, such as silica gel and precipitated calcium carbonate (PCC).
Additionally, precipitated silicas intended for dentifrices require comminution in order to reduce the particle size of the dried precipitated silica product down to a size that does not feel gritty in the mouth of a dentifrice user, while, on the other hand, not being so small as to lack sufficient polishing or thickening action. That is, in conventional practice, the median particle size of the silica in the reactor formed by acidulation of a metal silicate is too large for dentifrice applications and the like. To comminute silica particulates, grinding and milling equipment are used, such as a hammer or a pendulum mill used in one or multiple passes, and fine grinding has been performed, for example, by fluid energy or air-jet mill.
It has been found previously that narrow particle size distribution of comminuted precipitated silica materials provides very effective cleaning, polishing, and abrasion benefits for dental applications. Prior methods of precipitated silica production have proven effective to a certain extent for producing excellent materials for such purposes; however, it has also been found that such expensive spray/flash drying processes produce silica materials prior to comminution that exhibit an appearance and configuration that makes uniform milling for narrow particle size distribution results problematic. At the very least, such production methods result in a significant and appreciable amount of silica particles that exhibit particle sizes too great for effective utilization within such dental cleaning applications. Hence, not only are costs inflated through incorporation of the aforementioned spray/flash drying production steps, but the need to separate the unacceptably high particle size silica particles from the resultant precipitated materials also contributes to costs and inefficiency within the overall manufacturing procedure.
Furthermore, it has been desirable from a consumer aesthetic perspective to provide clear formulations of dentifrices within the pertinent market. Generally, silica particles are opaque in appearance and thus create difficulty when attempting to generate a clear or transparent gel or liquid toothpaste composition. It has been found in the past that certain manufacturing schemes could facilitate production of certain relatively high light transmittance precipitated silica materials to permit introduction of such a highly desired clear and/or transparent resultant dentifrice within the industry. However, even with such a possibility available, the highest transmittance level reliably provided by abrasive silicas was found to be at or around 70–75% on a reliable, consistent manufacturing basis. Thus, there remains the potential for significant improvement in this area with the evident possibility of providing greater amounts of effective silica particles (preferably exhibiting the above-discussed narrow particle size range for maximum dental abrasion and cleaning) within clear dentifrice gels, pastes, and/or liquids.
Additionally, the prevalent manufacturing methods for abrasive dental silicas have produced resultant materials that exhibit certain properties that, although not damaging or dangerous, leave room for improvement. Such properties include a propensity for dust generation from powdered silicas made via a process including at least one spray/flash drying step; difficulty in providing structural integrity in terms of consistently producing controlled low-structure silicas due to the number of process steps required for powdered silica manufacture; and suspect reliability in terms of reproducible narrow ranges of pellicle cleaning ratios and/or radiation dentin abrasion characteristics without potentially resorting to increased costs through separation methods of unwanted particle size silica materials.
Other issues that have arisen with such typically and traditionally produced silica and/or silicate materials include the need to ensure that pre-spray dried materials exhibit sufficiently low viscosities (and thus are relatively low in solids content) to prevent clogging of atomizers and other spray-drying equipment. The costs involved with such assurances may create increases to the silica and/or silicate manufacturer and/or dentifrice (or other type of composition if silicates are involved) producer that may be passed on to the consumer. Thus, the ability to overcome at least some of these issues, as well as provide suitable silica materials in terms of packing density (for storage and shipment), flowability (to facilitate transfer during incorporation into dentifrice formulations, at least), and, as noted above, a proper narrow particle size distribution, all without incurring increased manufacturing, additive, and/or separation costs, are all desirable for an improved production method for silicas and/or silicates, and, in particular, abrasive dental silica materials.
Therefore, a manufacturing method for precipitated silica and/or silicate materials that can provide not only lower costs and greater production efficiencies to the producer and highly effective cleaning and abrasion levels, or thickening capabilities (all due to easily produced narrow particle size ranges without waste particles outside such a range requiring sequestration and removal)(for silicas), or similar or other properties in terms of silicate materials, with possible higher light transmittance properties would be of great benefit both to the pertinent industry and to the aesthetic tastes of consumers. Unfortunately, to date, there have been no such advancements within the precipitated silica and/or silicate production industry.